Multi-level interconnection CMOS devices with SOG

ABSTRACT

A method of manufacturing a semiconductor wafer, which includes performing a first metallization to deposit a first layer of interconnect material on a substrate, etching the interconnect material to form interconnect tracks, depositing a first low temperature dielectric layer over the interconnect tracks, planarizing the first low temperature dielectric layer with quasi-inorganic or inorganic spin-on glass by a non-etchback process, depositing a second low temperature dielectric layer over the spin-on glass, etching via holes through the dielectric and spin-on glass layers to reach the tracks of the first interconnect layer, performing an in-situ desorption of physically and chemically bonded water vapour in a dry environment at a temperature of at least 400° C. and not more than 550° C. for a time sufficient to obtain a negligible desorption rate, the temperature exceeding by at least 25° C. the temperature to which the surface of the wafer will be exposed during a subsequent metallization step, and performing the subsequent metallization step to deposit a second interconnect layer extending through the via holes to the first interconnect tracks without re-exposure of the wafer to ambient conditions, and keeping this wafer under vacuum. This technique permits the reliable use of inorganic or quasi-inorganic spin-on glasses in non batch type sputtering equipment.

This invention relates generally to the fabrication of semiconductorwafers, and more particularly to a method of manufacturing asemiconductor wafer, wherein at least one high-temperature resistantwater absorbant, organic or quasi-organic dielectric layer is formedbetween first and second layers of deposited interconnect materialinterconnected through vias opened in the dielectric layer.

Spin-on glass (SOG) is a proprietary liquid solution containing silicate(purely inorganic spin-on glasses) or siloxane (quasi-inorganic spin-onglasses) based monomers diluted in various kinds of solvents oralcohols. It is mainly employed as a planarizing medium in themanufacture of semiconductor wafers to fill in trenches formed duringthe fabrication process, which involves depositing a plurality oflayers, some of which are partially etched during the fabricationprocess. During coating and curing, monomers are polymerized bycondensation of silanol, •SiOH, and ethoxy groups, •SiOC₂ H₅, andrelease water vapour, ethanol, and other by-products according to thefollowing scheme: ##STR1##

Polymerization of the SOG solution ceases when the distance betweenneighboring silanol groups, •SiOH, becomes too large or when too muchby-product, such as water vapour, blocks the condensation mechanism.Heating is then needed to eliminate condensation by-products and permitfurther condensation, densification, and the formation of a hardinorganic or quasi-inorganic film, i.e. the SOG film.

The final density of the SOG films depends on many factors but isgenerally lower than the density of other inorganic or quasi-inorganicglasses deposited by other commonly used techniques like LPCVD or PECVD.This lower density is due to the presence of many pores in the SOG film,which cause high conductance channeling paths between the film surfaceand its bulk. These pores permit the adsorbed gas molecules present onthe SOG film surface to continuously and rapidly diffuse through thebulk of the film and to rapidly connect physically to the glass byforming low energy (<0.1 eV) Van der Waals bonds (drawn as "• •" in thefollowing scheme); the gas molecules are rapidly absorbed physically bythe SOG film network.

Some gases, such water vapour, have molecules that can slowly form highenergy (>0.1 eV) chemical bonds (drawn as "•" in the following scheme)with the SOG film network by forming a pair of silanol groups, •SiOH.Water vapour molecules are slowly chemically absorbed by the SOG filmnetwork. ##STR2##

This slow chemical absorption of water vapour by the SOG film isparticularly efficient if the SOG solution contains phosphorusorganometallic molecules, which give very efficient water vapourgettering due to the presence of phosphorus-oxygen double bounds in theSOG film, as shown by the following scheme: ##STR3##

This gettering of water vapour by absorption can be verified by infraredspectroscopy by monitoring the behaviour of these P:O bonds and otherbonds in the presence of water vapour [see for example, R. M. Levin, J.Electrochem. Soc., Vol. 129, No. 8, pp. 1765-1770; M. Noyori and Y.Nakata, J. Electrochem. Soc., Vol. 131, No. 5, pp. 1109-1114].

As a reference, the following tables give the approximate location ofthe infrared absorption bands associated with the different inter-atomicvibrations of some selected bonds associated with the interaction ofwater vapour with phosphorus doped inorganic silicate thin films:

    ______________________________________                                                               APPROX.                                                                       WAVE                                                                          NUMBER                                                                        [cm.sup.-1 ]                                           ______________________________________                                        RELATED TO "OXYGEN-HYDROGEN" VIBRATIONS                                       in phase stretching of O--H in "H.OH"                                                                  3350                                                 in phase stretching of O--H in "H.OSi"                                                                 3650                                                 in phase stretching of O--H in "H.OP"                                                                  3700                                                 RELATED TO "OXYGEN-SILICON" VIBRATIONS                                        rocking of Si--O--Si      460                                                 banding of Si--O--Si      810                                                 in phase stretching of O--Si in "Si.OH"                                                                 930                                                 in phase stretching of O--Si in "Si.OSi"                                                               1070                                                 in phase stretching of O--Si in "Si.OP"                                                                1090                                                 out of phase stretching of O--Si in "Si.OSi"                                                           1140                                                 RELATED TO "OXYGEN-PHOSPHORUS" VIBRATIONS                                     in phase stretching of O--P in "P:O"                                                                   1280                                                 in phase stretching of O--P in "P.O"                                                                   1070                                                 in phase stretching of O--P in "P.OH"                                                                   950                                                 in phase stretching of O--P in "P.OSi"                                                                 1160                                                 in phase stretching of O--P in "P.OP"                                                                  1200                                                 ______________________________________                                    

According to the mechanism discussed above, adsorbed water vapourmolecules rapidly diffuse in the pores of the SOG, become rapidlyphysically absorbed (3350 cm⁻¹), and slowly become chemically absorbedby --P:O bonds (1280 cm⁻¹) and --Si•O•Si-- bonds (460, 810, 1070 and1140 cm⁻¹) to form respectively --P•OH (950 and 3700 cm⁻¹) and --Si•OH(930 and 3650 cm⁻¹) groups.

The same mechanism is observed for glasses other than spin-on glassesand has been reported for low temperature LPCVD and PECVD silicates. Itis believed that this mechanism is universal and should be observed forany type of glass. It is also believed that the lower the film porosity,implying less pores and smaller conductance from the film surface to itsbulk, the slower the water vapour channeling, its physical absorptionand chemical absorption.

Published literature [S. R. Hofstein, IEEE Trans. Electron Devices, Vol.ED-14, No. 11, pp. 749-759; M. Noyori, T. Ishihara and H. Higuchi,IEEE/PROC. IRPS 1982, pp. 113-121; N. Lifshitz and G. Smolinsky, J.Electrochem. Soc., Vol. 136, No. 8, pp. 2335-2340; H. G. Tompkins and C.Tracy, J. Electrochem. Soc., Vol. 136, No. 8, pp. 2331-2335] shows thetime dependance of water vapour absorption by porous glasses: there is arapid physical absorption, characterized by a t^(1/2) behaviour, and aslow chemical absorption, characterized by a t^(1/4) behaviour.According to these models, the less porous the material, the slower thediffusion through residual pores and the slower the physical as well aschemical absorption rates, down to a limit which represents diffusionand absorption rates associated with a perfectly packed amorphous glass.

Our Canadian patent application no. 2,017,720, filed on May 29, 1990discloses that these physical and chemical reactions involving watervapour and •P:O or •SiOSi• bonds are reversible at a relatively lowtemperature. The •POH (950 and 3700 cm⁻¹) and •SiOH (930 and 3650 cm⁻¹)groups disappear while •P:O bonds (1280 cm⁻¹) and •SiOSi bonds (460,810, 1070 and 1140 cm⁻¹) re-appear if the film is exposed to a dryambient or preferably a vacuum at temperature of about 400° to 500° C.for a time sufficiently long to permit:

(1) Slow chemical desorption, or destruction of chemical bonds with theglass network

(2) Rapid physical desorption, or destruction of the Van der Waals bondswith the glass network

(3) Rapid diffusion from the bulk to the surface through adjacent pores

(4) Rapid desorption from the surface.

A recently published paper [J. N. Cox, G. Shergill and M. Rose, PROC.VMIC, 1990, pp. 419-424] proves that this water desorption mechanism andmost of the adsorbed water vapour at the surface is readily eliminatedduring pump-down at room temperature. It also establishes that vacuumheating at about 150° C. is sufficient to quickly remove physicallyabsorbed water vapour, and that a higher temperature vacuum heating atabout 450° to 500° C. is necessary to completely eliminate chemicallyabsorbed water vapour.

Another recently published paper [R. A. M. Wolters, PROC. VMIC, 1990,pp. 447-449.] shows water vapour, H₂ O, carbon monoxide, CO, and carbondioxide, CO₂, desorption rates from a purely inorganic phosphorus dopedspin-on glass, Tokyo Ohka Kogyo OCD-2P-48316-SG. These gases wereadsorbed and absorbed during cool-down from a cure cycle of 425° C. andalso during storage. A heat treatment at a temperature of about 150° C.is very suitable to insure rapid desorption of physically bonded watervapour, and that a higher temperature heat treatment at about 425° to450° C. is necessary to insure desorption of chemically bonded water

If the glass is too porous, as in the case of spin-on glasses and veryporous LPCVD and PECVD glasses, its re-exposure to moist air causesextremely rapid readsorption of water vapour at the film surface, rapiddiffusion from pore to pore, rapid physical reabsorption and chemicalreabsorption that is so quick that misunderstanding of film quality,when verified by ex-situ analysis techniques, can happen.

This is the case for spin-on glass. G. Smolinsky, a worldwide spin-onglass expert from AT&T [N. Lifshitz and G. Smolinsky, ElectrochemicalSociety short course "Spin-on dielectrics for state-of-the-art VLSIapplications", Seattle, Wash., Oct. 14, 1990.], claims that spin-onglasses cannot be used in the manufacturing of high density and highperformance multi-level metallization CMOS devices because residualsilanol groups, •SiOH, cannot be eliminated without a temperaturetreatment that exceeds about 800° C., as verified by infraredspectroscopy, triangular voltage sweep, and vias chain measurementtechniques.

In fact, it is shown in our recent copending patent application No.2,017,720, filed on May 29, 1990 that residual water vapour, which ischemically absorbed as •POH or •SiOH groups, is eliminated by a muchlower temperature heat treatment of about 425° to 450° C., if undervacuum. It is also proven in the same patent application that a veryshort exposure of the order of 30 minutes to ambient air is long enoughto permit adsorption, physical absorption and chemical absorption ofwater vapour by SOG, thus explaining Smolinsky's misinterpretation.

It is also known that a heat treatment from 425° C. to more than about800° C. will increase the density of an SOG film by about 10%, thusclosing the pores at high temperature. The high temperature exposed SOG,and the low temperature exposed SOG, do not show residual physically orchemically absorbed water vapour when under vacuum, but re-exposure ofthe latter to the ambient will cause a quick water vapour reabsorptionwhile the former will not show "short time" reabsorption due to thesmall amount of pores.

In the fabrication of semiconductor wafers using a non-etchback SOGprocess, layers of interconnect material and dielectric are built up,with the dielectric planarized by the SOG. When the vias are opened up,the SOG comes into contact with the interconnect material depositedtherein. Since typical temperatures do not exceed 500° C. for this partof the process in order to prevent growth of hillocks and stressmigration problems, the SOG is generally porous and can absorb watervapour.

Since the cure temperature of the SOG used to smooth these glasses isalso limited to less than 500° C., and since it will be even more porousthan the surrounding glass, it will physically adsorb more water vapouras bonded H₂ O and chemically absorb more of it as •SiOH groups than itsequivalent volume of slightly porous PECVD or LPCVD glass. In the sameway, if the SOG and LPCVD or PECVD glasses incorporate phosphorus, watervapour can also be chemically absorbed as •POH groups.

The presence of water vapour in these dielectrics (LPCVD or PECVD glassand SOG) can cause serious manufacturing problems such as viaspoisoning, dielectric cracking and blistering, as well as reliabilityproblems of the completed device in the field. It is therefore importantto prevent rapid adsorption, rapid physical absorption and slow chemicalabsorption by these two dielectrics during device manufacturing toprevent these problems and insure device protection by gettering of thewater vapour by the glass and the SOG.

Our copending patent application No. 2,017,720, filed May 29, 1990,shows that rapid adsorption, rapid physical absorption and slow chemicalabsorption of water vapour are reversible at relatively lowtemperatures, lower than about 500° C., and that the use of a batchequipment for the deposition of the top LPCVD or PECVD glass is veryefficient for removing adsorbed, physically absorbed and chemicallyabsorbed water vapour in the SOG before capping it with a dense LPCVD orPECVD glass which prevents water vapour readsorption and reabsorption bythe SOG.

This rapid readsorption, rapid physical reabsorption and slow chemicalreabsorption of water vapour are prevented until the top protectivedielectric is opened when etching via contacts to the first level ofinterconnect material. Since the SOG pores are under vacuum (LPCVD andPECVD are low pressure processes), SOG will very rapidly adsorb andphysically absorb water vapour and other atmospheric gases (N₂, O₂, CO,CO₂) when exposed to air or any non-vacuum ambient. Chemical absorptionwill slowly occur.

The elimination of readsorbed and reabsorbed water vapour and ambientgases should be done before the metallization of the via holes in thedielectric which isolates first and second levels of interconnect toprevent the previously mentioned yield and reliability problems, as wellas vias poisoning, vias corrosion, metallization step coverage problemsin these vias, stress migration and electromigration problems.

After desorption of water vapour from the SOG, it is possible to depositthe second level of interconnect material. This metal based interconnectmaterial covers the top, sidewall and bottom of the via and, incombination with the second layer of dense LPCVD or PECVD dielectric,protects the SOG against readsorption and reabsorption of water vapourand other ambient gases.

The resulting device then incorporates an integrated gettering material,the dry SOG, that will ensure an improved reliability against watervapour penetration to the active device transistors during highlyaccelerated stress testing (HAST), temperature and humidity bias testing(THB), other reliability testing, or simply in the field.

As disclosed in our recent copending patent application referred toabove, it is possible to ensure desorption of adsorbed, physicallyabsorbed and chemically absorbed water vapour and to leave SOG poresunder vacuum before capping it by the deposition of the second layer ofdense and protective LPCVD or PECVD dielectric.

Exposure of the SOG layer to ambient occurs after opening the via andwater vapour readsorption and physical reabsorption occurs rapidly whilechemical readsorption occurs more slowly.

Single wafer sputtering apparatus is required to ensure: highthroughput, high uniformity, precisely controlled high temperature andhigh bias metallization of aluminium alloys and Ti and W containingalloys and compounds.

To maintain a production compatible throughput of a non etch-backspin-on glass process for the interconnection of high performance andhigh density multi-level metallization CMOS devices, it is necessary toprovide in-situ desorption of adsorbed, physically absorbed andchemically absorbed water vapour from SOG before depositing the secondlevel of interconnect material with single wafer sputtering equipments.

In the prior art total and partial etch-back spin-on glass processes arewidely used for the interconnection of CMOS devices which incorporateSOG over the first level of temperature sensitive interconnect material.In these etch-back processes, SOG is totally etched away in the areaover the first level of interconnect material and is then absent inareas where vias are formed. This prevents water vapour readsorption andreadsorption into the vias, but does not permit interconnection of highperformance and high density multi-level metallization CMOS devicesbecause of the tight process window of this complex process.

Dual via etch, non etch-back spin-on glasses processes have beenproposed, which use a first oversized via hole etch, then the depositionof a conformal, protective layer to cover the SOG present on thesidewall of the via, and then the etch of a second correctly sized viahole etch that prevents the exposure of SOG. This extremely precise viaetch sequence is not suitable for the interconnection of highperformance and high density multi-level metallization CMOS devicesbecause of the restriction of smaller and smaller via sizes.

Non etch-back, quasi-inorganic spin-on glass processes have beenproposed. These processes use methyl, •CH₃, or phenyl, •C₆ H₅,containing quasi-inorganic SOG that permit thicker coatings of SOGwithout cracking. These processes suffer from many drawbacks: Firstly,they are not stable at temperatures exceeding about 450° C. and, forthat reason, prohibit the use of very attractive high temperatureprocesses, such as LPCVD tungsten and high temperature sputtering forvia filling and step coverage improvement respectively. Secondly, theyare readily destroyed by oxygen, O₂, plasma treatments which are neededfor dry photoresist stripping. These SOGs contribute to the formation ofa carbon based residue at the bottom of the etched via which can causehigh specific via contact resistance, and they can cause field inversionproblems to sensitive CMOS devices because of the presence of theseorganic bonds.

The use of partial etch-back or non etch-back quasi-organic SOGprocesses is not suitable for the interconnection of high performanceand high density multi-level metallization CMOS devices.

Non etch-back, purely inorganic spin-on glass processes have beenproposed. These processes use batch type metallization equipment andvery short delays between via etching, post via etch photoresiststripping, post stripping inspection, and via metallization itself. Theyuse tightly controlled ambient dry boxes for wafer storage if extendedperiods are needed between these steps. This avoids slow chemicalabsorption of water vapour by the exposed SOG.

Just before entering the sputtering equipment, a pre-metallization watervapour desorption step may be performed in an independent vacuum or dryambient batch system, for an extended period of more than about 30minutes and at temperatures as high as about 500° C. A quick re-exposureto ambient air will cause surface reabsorption and physical reabsorptionof water vapour, while chemical reabsorption is prevented due to a tooshort re-exposure time.

The rate of reabsorption, physical reabsorption and chemicalreabsorption of water vapour can be evaluated by monitoring the stressbehaviour of spin-on glass wafers that are exposed to air followingremoval from the batch LPCVD equipment after an in-situ cure and beforedeposition of its protective film. This is possible since water vapourfilling the pores under vacuum causes the film to expand and generates acompressive stress; the stress of the film, initially under tensilestress, will become less and less tensile and may become compressive aswater vapour is physically absorbed.

This fast readsorption and physical reabsorption causes the film torapidly change its stress toward compression. Inversely, the slowchemical reabsorption causes the film to slowly change its stress towardtension. The observation of the SOG stress behaviour with time ofcontact with ambient air gives information on the rate of adsorption,physical absorption and chemical absorption.

Water vapour is adsorbed and physically absorbed by SOG within the firstfive hours and is continuously and slowly chemically absorbed; thelonger the SOG film exposure to ambient air, the more water vapour ischemically absorbed.

The amount of chemically absorbed water vapour is also a function of theambient air relative humidity. This is demonstrated by the followinggraph which correlates the equilibrium stress and the ambient airrelative humidity:

A non etch-back purely inorganic spin-on glass process can be carriedout using standard metallization equipment with:

a) The specification of short delays in ambient air between via etching,post via etch photoresist stripping, post stripping inspection, and viametallization itself. These delays must be less than about five hours intotal to prevent water vapour being chemically absorbed by the spin-onglass.

2) The specification of the use of tightly controlled ambient dry boxesfor wafer storage if extended periods are needed between these steps.This avoids slow chemical absorption of water vapour by the exposed SOGby imposing a very dry nitrogen or inert gas ambient.

3) The specification of an ex-situ pre-metallization water vapourdesorption step in an independent vacuum or dry ambient batch systemmust be for an extended period of more than about 30 minutes and at atemperature higher than about 400° C. but lower than about 530° C., justbefore entering the sputtering equipment. A quick re-exposure to ambientair of less than about one (1) hour before entering wafers in thesputtering equipment vacuum load-lock causes surface reabsorption andphysical reabsorption of water vapour but prevent its chemicalreabsorption will not occur to a too short re-exposure time.

4) The specification of an in-situ pre-metallization descrption step ofadsorbed and physically absorbed water vapour in the load-lock or in themain chamber of the sputtering system. This is done at relatively lowtemperatures, typically between 250° and 400° C.

As a result, the descrption efficiency is function of the history of thewafer and its ambient air exposure time. This process is wafer historysensitive and residual water vapour in the wafer is not alwaysnegligible, thus causing the previously mentioned yield and reliabilityproblem.

The article entitle "Application of Surface Reformed Thick Spin-on-Glassto MOS Device Planarization" (J. Electrochem. Soc., vol 137, no. 4,April 1990, pages 1212-1218; Shimida et al.) describes a novelpost-deposition treatment known as reactive glass stabilization (RGS).This article is, however, only concerned with organic SOGs and supportsthe conventional view that inorganic or quasi-inorganic SOGs areincapable of forming films thicker than 0.2μ due to their inherentfragility and brittleness, which causes cracking during baking.

An object of this invention is to provide an quasi-inorganic orinorganic planarization layer having improved cracking characteristics.

According to the present invention there is provided in a method ofmanufacturing a semiconductor wafer, wherein at least onehigh-temperature resistant water absorbant, organic, inorganic orquasi-inorganic dielectric layer is formed between first and secondlayers of deposited interconnect material interconnected through viasopened in said dielectric layer, the improvement wherein an in-situdesorption of physically and chemically water vapour in a dryenvironment at a temperature of at least 400° C. and not more than 550°C. for a time sufficient to obtain a negligible desorption rate, saidtemperature exceeding by at least 25° C. the temperature to which thesurface of the wafer will be exposed during a subsequent metallizationstep to form said second interconnect layer, etching via holes throughsaid dielectric layer said first interconnect layer, and performing saidsubsequent metallization step to deposit said second interconnect layerextending through said via holes to said first interconnect tracks whilemaintaining said dry environment, said subsequent etching andmetallization steps after said desorption step being performed withoutre-exposure of the wafer to ambient conditions.

In-situ in this context means that there is no ambient re-exposure ofthe wafer after the desorption step is completed.

While the organic, inorganic or quasi-inorganic dielectric layer isprefereably inorganic or quasi-inorganic spin-on glass, the invention isalso applicable to polyimide used as a dielectric over the first levelof metal interconnect, or any other high temperature (>400° C.)resistant organic dielectric.

This in-situ desorption can be carried out in the sputtering systemitself, or in its load-lock. The resulting process, its yield and thereliability of the completed devices are not as sensitive to the wafers'history.

At any temperature, slow chemical desorption of water vapour fromphosphorus or silicon is limited by the condensation rate of neighboring•POH or •SiOH groups, which depend on the •POH or •SiOH bond strengthand statistical distances between neighboring •POH or •SiOH groups.

As condensation between neighboring groups occurs, the distance betweenresidual groups increases and water vapour desorption rate eventuallybecomes negligible at that temperature. An increase of the temperatureenables •POH or •SiOH groups at larger separation to enter intocondensation reactions; the outgassing rate increases and eventuallydrops at that new temperature. If the new temperature is high enough,condensation of most if not all of the available •POH and •SiOH groupsoccurs and the SOG film becomes free from water vapour.

The chemical desorption temperature must exceed about 400° C. to ensuresubstantial condensation of these groups. If 400° C. is chosen as thedesorption temperature, water vapour desorption will eventually drop toa negligible value as the condensation becomes more and more difficult.If the metallization step causes the substrate temperature to exceed thewater vapour desorption temperature (400° C. in that case), viapoisoning, stress migration, electro-migration, step coverage and otherproblems will occur.

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings in which:

FIGS. 1 and 2 show respectively the infrared spectra of a dryphosphosilicate glass, and the infrared spectra of a water vapoursaturated phosphosilicate;

FIGS. 3, 4, 5, 6, 7, 8, 9, 10 and 11 show a sequence of steps in thefabrication of a semiconductor wafer;

FIG. 12 is a section through a back side heating plate and wafer in asputtering apparatus;

FIG. 13 is a graph showing the effect of back side argon pressure on hotplate heat transfer to the wafer; and

FIG. 14 is a graph showing the effect of varying back side argonpressure on hot plate heat transfer efficiency.

The effect of exposure of dry phosphorus doped inorganic silicate SOGthin films to water vapour can be seen in FIGS. 1 and 2.

Reference will now be made to FIGS. 3 to 11, which show the fabricationsteps in a typical wafer fabrication process.

The non etch-back SOG process used in the fabrication of interconnectionlayers for high performance, high density multi-level metallization CMOSdevices typically uses the following sequence shown in FIGS. 3 to 11.

A first level of temperature sensitive interconnect material 1 (FIG. 3),typically an aluminum alloy, combined or not with one or more layers ofa refractory metal, typically a titanium-tungsten compound, is depositedin single wafer sputtering apparatus.

This first level of interconnect material 1 is plasma etched 1a, 1b(FIG. 4), typically in various chlorine and fluorine containingdischarges, using standard photolithography techniques.

A first layer of relatively low temperature dielectric 2 (FIG. 5) isdeposited, typically at a temperature lower than about 450° C., bysingle wafer PECVD or LPCVD equipments, over the previously etchedinterconnect tracks.

The phosphorus-doped inorganic SOG solution 3 (FIG. 6) is applied,typically in one or many coats by a single wafer SOG processor, over thefirst layer of dielectric 1. Being liquid, it fills crevices and italmost absent over high topography. Relatively low temperature heattreatment, typically less than about 500° C., is applied to cure anddensify the SOG to form a hard and smooth glass. The resulting SOG filmis not etched back.

A second layer of relatively low temperature dielectric 4 (FIG. 7) isthen deposited, typically by single wafer PECVD or LPCVD equipments,over the cured SOG.

Via holes 5 (FIG. 8) are plasma etched, typically by single waferfluorine containing discharges, to reach the tracks of the first levelof interconnect.

A second level of temperature sensitive interconnect material 8 (FIG.9), similar to the one used for the first level, is typically depositedby single wafer sputtering equipment.

The second level of interconnect material 8 is etched similarly to thefirst level (FIG. 10).

The above sequence is repeated many times if more than two levels oftemperature sensitive interconnect material are needed.

A top protective layer 9 is then deposited (FIG. 11).

In accordance with the invention this process is carried out in nonbatch type sputtering apparatus or in a load-lock thereof.

After opening the vias water vapour desorption is carried out at atemperature that should exceed 400° C. to ensure substantial chemicaldesorption by •POH and •SiOH condensation, exceed by at least 25° C. thehighest temperature that the surface of the wafer will see during themetallization of these vias, not exceed 550° C. so as to minimizehillock growth, junction spiking and film cracking problems, and beapplied for a sufficiently long time, at least 30 minutes, to obtain anegligible water vapour desorption rate at that desorption temperature.

It is important to precisely control temperature of the wafer. Becauseof their lack of control over desorption temperatures, infrared heatedand other thermal batch desorption systems are not suitable. A back sidesingle wafer desorption hot plate 10 (FIG. 12), is preferred. Beingunder vacuum in sputtering apparatus 12, it is difficult to ensure agood thermal contact between this back side hot plate 10 and the wafer11. By using a high pressure argon flow 13 on the back side of the wafer10, between the wafer and the hot plate 10, it is possible to improvethe heat transfer between the temperature controlled hot plate 10 andthe wafer 11, thus ensuring a well controlled and reproducibledesorption temperature.

The effect of back side argon pressure on hot plate heat transfer to thewafer can be seen in FIG. 13.

Without argon pressurization of the gap between the back side of thewafer 11 and the hot plate 10, it is not possible to heat the wafer tomore than about 325° C. because of poor thermal contact and wafer heatdissipation by infrared radiation. This maximum wafer temperature is notsuitable for desorption of chemically absorbed water vapour, which needsa minimum of 400° C. with an optimum temperature to be chosen between400° and 500° C. The long response time associated with the wafertemperature rise is not suitable for a single wafer desorption approachbecause of a low throughput.

The use of an argon pressurized, back side wafer hot plate 10 gives amuch better thermal contact between the hot plate and the wafer. Thispermits the wafer to reach, within less than 30 seconds, a temperaturethat is very close to the chuck temperature. Wafer temperatures willnever equal chuck temperatures because of a non perfect thermal contactand an important infrared heat dissipation by the wafer at temperatureexceeding 400° C. By clamping the wafer on this hot plate, it ispossible to vary the pressure behind the wafer, as to vary the heattransfer efficiency, by adjusting the argon flow behind the wafer (FIG.5).

This single wafer hot plate is used with a precise back side argonpressure that is adjusted between about 0.2 and 20 Torr to provideefficient heat transfer between the hot plate and the wafer, in a vacuumchamber which is maintained at a pressure lower than the back side waferpressure.

The hot plate temperature and wafer back side argon pressure areselected to provide a quick ramp-up of the wafer temperature to theequilibrium wafer temperature within less than 45 seconds. The hot platetemperature and the wafer back side argon pressure are also selected toensure a wafer equilibrium temperature that is higher than 400° C. andthat exceeds by at least 25° C. the highest sputtering temperature thatwill be use to deposit the interconnect materials.

An infrared lamp can be used to estimate desorption time of adsorbed,physically absorbed as well as chemically absorbed water vapour attemperatures that permit chemical desorption to occur. A desorption timeof at least two minutes at the wafer equilibrium temperature is enoughto ensure a complete desorption of wafer vapour.

The described process, which uses an argon pressurized back side waferhot plate, can ensure an in-situ, high throughput, and preciselycontrolled single wafer desorption of adsorbed, physically absorbed andchemically absorbed wafer vapour before sputtering interconnectmaterials. In-situ, in the context of this application, means a chamberconfiguration that prevents an exposure to ambient air between thisdesorption step and the sputtering of the interconnect materials, eitherin the same chamber or an independent one.

The described technique permits the reduction of die size of multi-levelinterconnect devices by reducing via dimensions due to an improved viadesorption efficiency which reduces interconnect materials oxidation andspecific contact resistance. It also permits better yields and morereliable multi-level interconnect CMOS devices. It prevents thespecification of tightly controlled delays between manufacturing steps,the use of well controlled dry boxes for the storage of partly processeddevices, and improves the process flow by reducing manufacturingconstraints.

Thus, in summary the process is carried out in single wafer sputteringapparatus to ensure high throughput, high uniformity and preciselycontrolled high temperature and high bias metallization of aluminumalloys and Ti and W containing alloys and compounds,

The water vapour desorption should be done rapidly to maintain aproduction compatible throughput of a non etch-back spin-on glassprocess for the interconnection of high performance and high densitymulti-level metallization CMOS devices,

Adsorbed water vapour at the surface can be readily eliminated duringvacuum pump-down at room temperature,

Vacuum heating at about 150° C. is sufficient to quickly removephysically absorbed water vapour, and Vacuum heating at a temperatureexceeding about 400° C. permits desorption of chemically absorbed watervapour,

As neighboring •SiOH and •POH groups condensation occurs, thestatistical distance between residual groups increases and water vapourdesorption rate eventually becomes negligible, at that temperature.

A minimum desorption time of about two minutes is required, at thattemperature, to obtain a negligible water vapour desorption rate,

An increase of that temperature enables •POH or •SiOH groups at largerseparations to enter into condensation reactions, thus causing watervapour desorption rate to increase and eventually drop, at thatincreased temperature,

If the temperature is high enough, condensation of most if not all ofthe available •POH and •SiOH groups occurred and the SOG film becomeswater vapour free.

Independently of the desorption temperature, this temperature mustexceed by at least 25° C. the highest metallization temperature thatwill see the surface of the wafer during metallization. The waferdesorption temperature should not exceed about 550° C. to minimizehillock growth, junction spiking and films cracking problems,

This technique permits the reduction of die size of multi-levelinterconnect devices by reducing via dimensions due to an improved viadesorption efficiency which reduce interconnect materials oxidation andspecific contact resistance. It also permits the manufacturing of betteryield and more reliable multi-level interconnect CMOS devices, andprevents the specification of tightly controlled delays betweenmanufacturing steps, the use of well controlled dry boxes for thestorage of partly processed devices, and improves the process flow byreducing manufacturing constraints.

Although this invention is more beneficial to phosphorus doped inorganicSOG, it is also beneficial to: boron, arsenic, lead or other metal dopedinorganic SOGs and combinations of the above, to undoped inorganic SOG,undoped quasi-inorganic SOG which incorporate some --CH₃, --C₂ H₅ orsome others organic bonds or combinations of these, and to dopedquasi-organic SOG which incorporate some organic bonds as well asphosphorus, boron, arsenic, lead or other metal.

This technique is also applicable to polyimide used as a dielectric overthe first level of metal interconnect, or for any other high temperature(>400° C.) resistant organic dielectric.

This technique is also applicable to porous dielectrics deposited byvarious chemical vapour deposition at temperatures lower than 500° C.:low pressure chemical vapour deposition of dielectrics, LPCVD, plasmaenhanced chemical vapour deposition of dielectrics, PECVD, laserassisted chemical vapour deposition of dielectrics, LACVD, as well asphoto enhanced chemical vapour deposition of dielectrics, PhCVD. Thetechnique also applies to sputtered dielectrics as well as dielectricsdeposited by electron cyclotron resonance, ECR.

Although this technique is described for sputtered interconnectmaterials, it is also applicable to other deposition techniques forinterconnect materials: low pressure chemical vapour deposition ofinterconnects, plasma enhanced chemical vapour deposition ofinterconnects, laser assisted chemical vapour deposition ofinterconnects, photo enhanced chemical vapour deposition ofinterconnects, thermal evaporation, as well as electron beam evaporationof interconnects and electron cyclotron resonance deposition ofinterconnects.

While this technique has been described with reference to a single or atwo chamber sputtering system, in which the desorption is respectivelydone in-situ in the sputtering chamber itself, or in-situ in thedesorption load-lock from where the wafer is transported under vacuum tothe sputtering chamber, it also apply to cluster tools or otherintegrated multi-chamber systems for which wafer transport is performedunder vacuum between the various chambers.

Although this technique uses argon as the back side gas, it also applyto any other inert or reactive gas because the heat transfer is morefunction of the gas pressure than of the gas identity. Gas mixtures arealso possible.

It is possible to use a heating chuck that supports more than one wafer.In this case, the back side gas is injected between each individualwafer and the hot platen, thus insuring a uniform, fast, and wellcontrolled, multi-wafer desorption.

I claim:
 1. A method of manufacturing a semiconductor wafer, comprisingthe steps of:a) depositing a first layer of interconnect material on asubstrate; b) etching said interconnect material to form interconnecttracks; c) depositing a first low temperature dielectric layer over saidinterconnect tracks; d) planarizing said first low temperaturedielectric layer with a quasi-inorganic or inorganic spin-on glass by anon-etchback process; e) depositing a second low temperature dielectriclayer over said spin-on glass; f) etching via holes through saiddielectric and spin-on glass layers to reach the tracks of said firstinterconnect layer; g) performing a desorption of physically andchemically water vapour in a dry environment at a temperature of atleast 400° C. for at least 30 minutes; h) performing a subsequentmetallization step to deposit a second interconnect layer extendingthrough said via holes to said first interconnect tracks whilemaintaining said dry environment; and i) wherein said desorption iscarried out in situ at a temperature at least 25° C. higher than thetemperature at which said subsequent metallization step is carried outand not more than 550° C., and said subsequent metallization step aftersaid desorption step is performed without re-exposure of the wafer toambient conditions.
 2. A method of manufacturing a semiconductor waferas claimed in claim 1, characterized in that said wafer is heated duringsaid desorption step with a back side hot plate.
 3. A method ofmanufacturing a semiconductor wafer as claimed in claim 2, characterizedin that high pressure inert gas is caused to flow between said wafer andhot plate to improve heat transfer between the hot plate and the wafer.4. A method of manufacturing a semiconductor wafer as claimed in claim3, characterized in that said inert gas is argon.
 5. A method ofmanufacturing a semiconductor wafer as claimed in claim 4, characterizedin that the argon is maintained at a pressure between 0.2 and 20 Torr.6. A method of manufacturing a semiconductor wafer as claimed in claim5, characterized in that said desorption is carried out in a vacuummaintained at a pressure lower than the back side wafer pressure.
 7. Amethod of manufacturing a semiconductor wafer as claimed in claim 1,characterized in that said metallization is effected in a sputteringapparatus, and said desorption step is carried out in said sputteringapparatus or a load-lock thereof.
 8. A method of manufacturing asemiconductor wafer as claimed in claim 1, characterized in that saidspin-on glass is phosphorus-doped inorganic spin-on glass.
 9. A methodof manufacturing a semiconductor wafer as claimed in claim 1,characterized in that said time for performing said in-situ desorptionis at least thirty minutes.